1. Field of the Invention
The present invention relates to a thermoplastic polyurethane resin. The thermoplastic polyurethane resin possesses a hydroxyl group or hydroxyl groups at a side portion or side portions thereof so that the curing through the reactivity of such hydroxyl group or groups in a three-dimensional network structure can provide improved thermal resistant and solvent-resistant properties which are useful for adhesives, coating agents and binders.
Hydrophilic properties of the hydroxyl groups present at the side portions of a polyurethane resin or the tertiary amino groups present in the main molecular chain thereof can improve dispersibility of a pigment and an inorganic filler. Those polar groups also can serve as improving compatibility with other resins so that the thermoplastic polyurethane resins having such polar groups are useful particularly as binders, coatings and so on.
2. Brief Description of the Prior Art
Urethane resins are extensively used as adhesives, artificial hides or skins, paints, elastomers, coatings and binders. Various modes of employing the urethane resins are known. They can be roughly classified into the following groups: for example, the two-pack method in which a polyisocyanate is subjected to a reaction for chain prolongation or propagation or a cross-linking reaction for a network formation with an active hydrogen compound such as a low molecular weight polyol and, as necessary, a triol and so on; a moisture-curing one-can method in which a urethane prepolymer retaining therein a portion of the isocyanate groups formed by the reaction of a polyol with a polyisocyanate is reacted with moisture in the air; a block isocyanate one-can method in which a polyol is employed as a mixture with a polyisocyanate (block isocyanate) masked with a blocking agent; and the thermoplastic polyurethane method in which a linear high molecular weight polyurethane or a so-called thermoplastic polyurethane resin is used.
The two-pack method can provide a three-dimensional network structure after curing so that it can give favorable effects on various properties. The method, however, involves the mixing of two liquids which are both low molecular weight compounds so that the method has disadvantages that there is a limitation on operability in respect of pot life and so on and that an initial physical property is weak until the curing proceeds to some degree. As a product obtainable by the two-pack method has an initial tackiness in many cases, the step of winding it on itself or on something else in roll immediately after coating or the step of recoating on a previous coat may cause problems with drying and blocking.
The one-can method using the block isocyanate presents disadvantages that it requires high temperatures for curing so as to cause the blocking agent to be eliminated and that, where the blocking agent remains partially in the resulting resin, the agent will give an adverse effect on physical properties of the resin and cause an environmental pollution in association with the scattering of the agent. These disadvantages permit limited use of the resulting resin.
The method of curing in moisture may cause a problem with curing because the curing varies to a great extent with ambient conditions such as moisture, temperature and so on and because the generation of carbon dioxide may become a cause of bubbles. This problem also causes the resulting resin to permit a limited use for the resulting resin.
Different from those methods as hereinabove referred to, the thermoplastic polyurethane method can present advantages that the drying property for forming a film by the evaporation of a solvent immediately after the coating is favorable because of the use of a high molecular polyurethane resin dissolved in the solvent and that a pot life required for operation of the solution is unlimited. Conventional thermoplastic polyurethane method, however, presents the drawback that, as the film resulting from conventional thermoplastic polyurethane resin is not in a network structure, the film becomes dissolved or swollen in a particular solvent, whereby the film is rendered poor in solvent resistance. The method, however, requires improvement because the resin resulting from such polyurethane resin melts at temperatures above the softening point thereof and because it is poorer in heat resistance than crosslinked resins obtainable by methods other than such thermoplastic polyurethane method.
Thermoplastic polyurethane resins containing a terminal hydroxyl group or terminal hydroxyl groups, which may be employed particularly for adhesives, coating agents, binders for magnetic tapes and inks, and so on, are used in the form of a solution in a solvent such as a ketone, e.g., acetone, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone or the like; an aromatic hydrocarbon, e.g., toluene, isophorone or the like; an alcohol, e.g., isopropyl alcohol or the like; or a mixture thereof. Such thermoplastic polyurethane resins, however, have the drawbacks that a solubility thereof in the solvent as hereinabove referred to becomes poorer in instances where the concentration of urethane groups in the polyurethane resin is rendered high by increasing a ratio of a low molecular weight diol in the resulting polyurethane resin in order to provide enhanced heat resistance and solvent resistance of the resulting thermoplastic polyurethane resin. Accordingly, in this case, the use of a solvent having a strong action to solubilize the polyurethane resin and a high polarity is required. Representatives of such solvent may be enumerated by dimethyl formamide, tetrahydrofuran or the like. Such solvent, however, may cause the disadvantage that it will corrode surface portions of a base film, a coated object or the like on which the product containing such solvent is brought into contact, whereby wrinkles or crinkles are partially caused or, in some cases, the portions may become dissolved. Therefore, there is a limitation on improvement by increasing the concentration of the urethane groups in the resulting thermoplastic polyurethane resin to be used.
As a process for improving, in particular, heat resistance and solvent resistance of a thermoplastic polyurethane resin, there is known a procedure in which a polyisocyanate such as a commercially available product "Coronate L" (manufactured and sold by Nippon Polyurethane Industry Co., Ltd.) is added to the thermoplastic polyurethane resin containing a terminal hydroxyl group or terminal hydroxyl groups and then the resulting mixture is subjected to chain prolongation or propagation and crosslinking reactions. In this process, both the hydroxyl groups present at the terminals of the thermoplastic polyurethane resin chain and a urethane bond or bonds present in the molecular chain thereof are allowed to react with the polyisocyanate. In particular, the reaction of the urethane bond with the isocyanate group of the polyisocyanate, which is called an allophanate reaction, that is, a reaction for forming the allophanate bond, requires high temperatures and it cannot provide a sufficient network structure in the resulting polyurethane resin where the resin is used to form a coating.
Where conventional thermoplastic polyurethane resins are used for coating agents, binders for magnetic tapes and inks and the like, in which an inorganic filler or the like is dispersed or filled therein, workability at the time of coating, such as drying property, recoatability, curing velocity, liquid properties and the like is of great significance, in addition to various physical properties such as durability, weathering, adhesion to a base material and the like. On the top of those characteristic properties, thermoplastic polyurethane resins have been demanded to have desirable properties in respect of segregation, sedimentation, gloss on the surface of a cured product and the like. In particular, binders for magnetic tapes further require that the thermoplastic polyurethane resin to be used can provide favorable electromagnetic performance and properties resulting from the dispersibility of a pigment in the polyurethane resin. Conventional thermoplastic polyurethane resins, however, have the drawback that their action to disperse a pigment, an inorganic filler or the like is particularly poor so that thermoplastic polyurethane resins having particularly improved dispersion properties have been desired.
As a polyurethane resin possesses elastomeric properties and a resistance to wear to such an extent as other resins cannot provide, characteristic properties may be expected when such polyurethane resin is blended with the other resins. Conventional thermoplastic polyurethane resins, however, are less satisfactory in compatibility with the other resins.
As a method of improving the heat resistance and the solvent resistance of a thermoplastic polyurethane resin while retaining favorable drying properties and flexibility originating from the polyurethane resin, there is known a method of providing a three-dimensional network structure by subjecting the resin to crosslinking between the polyurethane molecule chains within a range in which the flexibility of the resulting resin is not impaired. This method permits the formation of a three-dimensional network structure in the resulting polyurethane resin by adding a curing agent likely to be reactive with a functional group to the thermoplastic polyurethane resin which has at least two functional groups which can be reacted with the curing agent.
Since the pigment, the inorganic filler or the like to be contained in coating agents, various binders and so on has adsorption water and/or chemically bonded water on its surface even when dried to a sufficient extent, consequently it shows a hydrophilic property and compatibility with a compound having a hydroxyl group, a carboxyl group, a sulfone group, a tertiary amino group, a quaternary amino group or the like. As a result, a dispersibility of the coating agents, various binders or the like is improved by adsorption through the aid of those groups.